[SCEV] Factor out utility for proving same sign of two SCEVs [nfc] (#170376)

This is a slightly different API than ConstantRange's
areInsensitiveToSignednessOfICmpPredicate. The only actual difference
(beyond naming) is the handling of empty ranges (i.e. unreachable code).
I wanted to keep the existing SCEV behavior for the unreachable code as
we should be folding that to poison, not reasoning about samesign. I
tried the other variant locally, and saw no test changes.

Author: preames

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -1078,6 +1078,9 @@ class ScalarEvolution {
   isKnownMultipleOf(const SCEV *S, uint64_t M,
                     SmallVectorImpl<const SCEVPredicate *> &Assumptions);
 
+  /// Return true if we know that S1 and S2 must have the same sign.
+  LLVM_ABI bool haveSameSign(const SCEV *S1, const SCEV *S2);
+
   /// Splits SCEV expression \p S into two SCEVs. One of them is obtained from
   /// \p S by substitution of all AddRec sub-expression related to loop \p L
   /// with initial value of that SCEV. The second is obtained from \p S by

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -11115,6 +11115,11 @@ bool ScalarEvolution::isKnownMultipleOf(
   return true;
 }
 
+bool ScalarEvolution::haveSameSign(const SCEV *S1, const SCEV *S2) {
+  return ((isKnownNonNegative(S1) && isKnownNonNegative(S2)) ||
+          (isKnownNegative(S1) && isKnownNegative(S2)));
+}
+
 std::pair<const SCEV *, const SCEV *>
 ScalarEvolution::SplitIntoInitAndPostInc(const Loop *L, const SCEV *S) {
   // Compute SCEV on entry of loop L.
@@ -12034,8 +12039,7 @@ bool ScalarEvolution::isImpliedCondBalancedTypes(
   if (IsSignFlippedPredicate(Pred, FoundPred)) {
     // Unsigned comparison is the same as signed comparison when both the
     // operands are non-negative or negative.
-    if ((isKnownNonNegative(FoundLHS) && isKnownNonNegative(FoundRHS)) ||
-        (isKnownNegative(FoundLHS) && isKnownNegative(FoundRHS)))
+    if (haveSameSign(FoundLHS, FoundRHS))
       return isImpliedCondOperands(Pred, LHS, RHS, FoundLHS, FoundRHS, CtxI);
     // Create local copies that we can freely swap and canonicalize our
     // conditions to "le/lt".